Large-screen Television Technology - Display Technologies - LCD Television

LCD Television

A pixel on an LCD consists of multiple layers of components: two polarizing filters, two glass plates with electrodes, and liquid crystal molecules. The liquid crystals are sandwiched between the glass plates and are in direct contact with the electrodes. The two polarizing filters are the outer layers in this structure. The polarity of one of these filters is oriented horizontally, while the polarity of the other filter is oriented vertically. The electrodes are treated with a layer of polymer to control the alignment of liquid crystal molecules in a particular direction. These rod-like molecules are arranged to match the horizontal orientation on one side and the vertical orientation on the other, giving the molecules a twisted, helical structure. Twisted nematic liquid crystals are naturally twisted, and are commonly used for LCD’s because they react predictably to temperature variation and electric current.

When the liquid crystal material is in its natural state, light passing through the first filter will be rotated (in terms of polarity) by the twisted molecule structure, which allows the light to pass through the second filter. When voltage is applied across the electrodes, the liquid crystal structure is untwisted to an extent determined by the amount of voltage. A sufficiently large voltage will cause the molecules to untwist completely, such that the polarity of any light passing through will not be rotated and will instead be perpendicular to the filter polarity. This filter will block the passage of light because of the difference in polarity orientation, and the resulting pixel will be black. The amount of light allowed to pass through at each pixel can be controlled by varying the corresponding voltage accordingly. In a color LCD each pixel consists of red, green, and blue subpixels, which require appropriate color filters in addition to the components mentioned previously. Each subpixel can be controlled individually to display a large range of possible colors for a particular pixel.

The electrodes on one side of the LCD are arranged in columns, while the electrodes on the other side are arranged in rows, forming a large matrix that controls every pixel. Each pixel is designated a unique row-column combination, and the pixel can be accessed by the control circuits using this combination. These circuits send charge down the appropriate row and column, effectively applying a voltage across the electrodes at a given pixel. Simple LCD’s such as those on digital watches can operate on what is called a passive-matrix structure, in which each pixel is addressed one at a time. This results in extremely slow response times and poor voltage control. A voltage applied to one pixel can cause the liquid crystals at surrounding pixels to untwist undesirably, resulting in fuzziness and poor contrast in this area of the image. LCD’s with high resolutions, such as large-screen LCD televisions, require an active-matrix structure. This structure is a matrix of thin-film transistors, each corresponding to one pixel on the display. The switching ability of the transistors allows each pixel to be accessed individually and precisely, without affecting nearby pixels. Each transistor also acts as a capacitor while leaking very little current, so it can effectively store the charge while the display is being refreshed.

The following are types of LC display technologies:

  • Twisted Nematic (TN): This type of display is the most common and makes use of twisted nematic-phase crystals, which have a natural helical structure and can be untwisted by an applied voltage to allow light to pass through. These displays have low production costs and fast response times but also limited viewing angles, and many have a limited color gamut that cannot take full advantage of advanced graphics cards. These limitations are due to variation in the angles of the liquid crystal molecules at different depths, restricting the angles at which light can leave the pixel.
  • In-Plane Switching (IPS): Unlike the electrode arrangement in traditional TN displays, the two electrodes corresponding to a pixel are both on the same glass plate and are parallel to each other. The liquid crystal molecules do not form a helical structure and instead are also parallel to each other. In its natural or "off" state, the molecule structure is arranged parallel to the glass plates and electrodes. Because the twisted molecule structure is not used in an IPS display, the angle at which light leaves a pixel is not as restricted, and therefore viewing angles and color reproduction are much improved compared to those of TN displays. However, IPS displays have slower response times. IPS displays also initially suffered from poor contrast ratios but has been significantly improved with the development of Advanced Super IPS (AS – IPS).
  • Multi-Domain Vertical Alignment (MVA): In this type of display the liquid crystals are naturally arranged perpendicular to the glass plates but can be rotated to control light passing through. There are also pyramid-like protrusions in the glass substrates to control the rotation of the liquid crystals such that the light is channeled at an angle with the glass plate. This technology results in wide viewing angles while boasting good contrast ratios and faster response times than those of TN and IPS displays. The major drawback is a reduction in brightness.
  • Patterned Vertical Alignment (PVA): This type of display is a variation of MVA and performs very similarly, but with much higher contrast ratios.

Read more about this topic:  Large-screen Television Technology, Display Technologies

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